Processes for converting lower oxygenates such as methanol and dimethyl ether (DME) to hydrocarbons are known and have become of great interest because they offer an attractive way of producing liquid hydrocarbon fuels, especially gasoline, from sources which are not petrochemical feeds. In particular, they provide a way by which methanol and DME can be converted to gasoline boiling components, olefins and aromatics. Olefins and aromatics are valuable chemical products and can serve as feeds for the production of numerous important chemicals and polymers. Because of the limited supply of competitive petroleum feeds, the opportunities to produce low cost olefins from petroleum feeds are limited. However, methanol may be readily obtained from coal by gasification to synthesis gas and conversion of the synthesis gas to methanol by well-established industrial processes. As an alternative, the methanol may be obtained from natural gas or biomass by other conventional processes.
Available technology to convert methanol and other lower oxygenates to hydrocarbon products utilizes a fixed bed process, such as the processes described in U.S. Pat. Nos. 3,998,899; 3,931,349 and 4,035,430. In the fixed bed process, the methanol is usually first subjected to a dehydrating step, using a catalyst such as gamma-alumina, to form an equilibrium mixture of methanol, DME and water. This mixture is then passed at elevated temperature and pressure over a catalyst for conversion to the hydrocarbon products which are mainly in the range of light gas to gasoline. The fixed bed process uses a recycle gas for temperature control and very large heat transfer to manage low quality heat, which results in high compression costs and a large heat exchange network. Typically, a fixed bed process is a multi-reactor, unsteady state operation, which requires a large bore valving system to control the process.
In contrast, direct cooling of the reactor in the fluidized bed process eliminates the need for recycle gas for temperature control, which simplifies the heat exchange. Further, the fluidized bed process with continuous catalyst regeneration is a steady state operation with constant product yield. Thus, the fluidized bed process requires lower capital costs and savings on operating expenses compared to the fixed bed process. However, current fluidized bed processes typically have a low product yield. For example, C5+ gasoline yield from a fluidized bed process ranges from 65 wt % to 75 wt % of hydrocarbons (HC), while the C5+ gasoline yield from a fixed bed process ranges from 80 wt % to 90 wt % of HC. Thus, an alkylation unit is usually required to increase C5+ gasoline yield in a fluidized bed process. Therefore, there is a need to provide fluidized bed processes for converting oxygenates to hydrocarbons with increased product yields and further, without the use of an alkylation unit.